Restore original intent of memset instcombine test

Restore original intent of memset instcombine test

Summary:
The original intent of test/Transforms/InstCombine/memset.ll was to test for lowering of llvm.memset into stores when the size of the memset is 1, 2, 4, or 8. Sometime between then and now the test has stopped testing for that, but remained passing due to testing for the absence of llvm.memset calls rather than the presence of store instructions. Right now this test ends up with an empty function body because the alloca is eliminated as safe-to-remove, which results in the llvm.memset calls's being eliminated due to their pointer args being undef; so it is not testing for conversion of llvm.memset into store instructions at all.

This change alters the test to verify that store instructions are created, and moves the target of the memset to an arg of the proc to avoid it being eliminated as unused.

Reviewers: anna, efriedma

Reviewed By: efriedma

Subscribers: efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D34642

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306681 91177308-0d34-0410-b5e6-96231b3b80d8